Electro-wetting is essentially the phenomenon whereby an electric field modifies the wetting behavior of a polar liquid in contact with a hydrophobic surface. By applying an electric field, a surface energy gradient is created in the polar liquid, which can be used to manipulate the liquid. In common applications, water is used as the polar liquid.
Recently, switchable elements have been proposed and demonstrated to operate based on the principle of electro-wetting. Such switchable elements typically consist of a closed cell that is filled with one part of water and one part of oil, although other liquids might also be used. The important characteristics are that the liquids are immiscible and that one of them is polar and or electrically conducting (e.g. water), while the other one is non-polar (e.g. oil). Other liquid parameters of importance, when the device is used as an optical element, are for example refractive index, melting point, transmission and density.
Since the liquids are immiscible, a well-defined interface will always be present between them. The inner surface of the cell generally comprises two separate surfaces, one that is hydrophobic and one that is non-hydrophobic. The hydrophobic surface will by nature reject the water, and by configuring the surfaces properly the spatial relationship between the liquids can be predetermined, i.e. the water is forced to a predetermined location opposite the hydrophobic surface. Consequently, also the interface between the two liquids can be predetermined.
Furthermore two electrodes are arranged in the cell, one address electrode arranged behind the hydrophobic and electrically insulating coating and one counter electrode in direct contact with or capacitively coupled to the conducting liquid. By applying a potential between the electrodes, an electric field is created across the insulating coating. The electric field gives rise to an electrostatic force that overrides the force of attraction exercised by the molecules in the conducting liquid and thus modifies the spatial relation between the liquids and consequently also the shape and position of the liquid interface. In effect, the liquid interface can be controlled by means of controlling the applied potential.
There are several known principles exploiting this mechanism and with which a cell can be controlled. According to a first principle the liquids are chosen to have different transmission properties. By changing the spatial distribution of the liquids, the transmission of the component is varied. According to a second principle, the liquids are chosen to have different indices of refraction. This turns the meniscus between the liquids into a lens having refractive properties that can be controlled by means of the electrode potential. Typically, this lens can be changed between a convex, light focusing state and a concave, light defocusing state. In this document an OFF-state refers to a condition, wherein an applied voltage between the electrodes is substantially zero. Further, and an ON-state refers to a condition, wherein an applied voltage causes a substantial change in spatial distribution of the liquids, compared to the OFF-state.
Electro wetting cells are further described in WO2002/099527, WO2003/069380 and WO2004/027489.
These electro-wetting elements can be arranged to work as different optical and other components, for example as motor, variable focus lenses, variable diaphragms, variable filters, gratings, beam deflectors, mechanical actuators and electro-wetting based displays. When these electro-wetting elements are used in portable devices, the power consumption and the actuation voltages of the device are of particular importance. If the actuation voltage is too high it might be necessary to include additional electronics to drive the device. Such additional electronics has a number of drawbacks, one being increased development and manufacturing costs of the device. Another drawback related to a high power consumption might be that batteries of the portable device need to be recharged so frequently, that use of the portable device is substantially restricted.